Cellulose esters are valuable polymers that are useful in many plastic, film, coating, and fiber applications. Cellulose esters (CEs) are typically synthesized by the reaction of cellulose with an anhydride or anhydrides corresponding to the desired ester group or groups, using the corresponding carboxylic acid as diluent and product solvent. Some of these ester groups can afterward be hydrolyzed to obtain a partially-esterified product. These partially substituted cellulose esters have great commercial value as opposed to their fully esterified counterparts, and find use in coatings, where their solubility and compatibility with co-resins and available hydroxyl group functionality, which facilitates crosslinking, are prized.
An important aspect in obtaining suitable cellulose esters has traditionally been maintaining molecular weight during the esterification process. A loss in molecular weight is associated with poor plastic properties and brittle films, a flexible film being the desired goal. Thus, it has long been recognized that in order to obtain a suitable chloroform-soluble (triacetate) cellulose ester, the acetylation process must not result in significant degradation, or lowering of the molecular weight, of the cellulose. See, for example, U.S. Pat. No.1,683,347.
When it was discovered that these early triacetate esters could be modified, via partial hydrolysis of the acetate groups, to obtain acetone-soluble cellulose acetate, maintaining a suitable molecular weight during hydrolysis remained critical. See, for example, U.S. Pat. No. 1,652,573. It was recognized as early as the 1930's that the amount of hydrochloric acid present in the reaction mixture during partial ester hydrolysis must be carefully controlled to avoid hydrolysis or breakdown of the cellulose acetate. See, for example, U.S. Pat. No.1,878,954.
Likewise, U.S. Pat. No. 2,129,052 advised that hydrolysis under severe conditions such as high temperature or high concentration of catalyst caused degradation of the cellulose, the resulting products being unsuitable for commercial use because of their low strength. U.S. Pat. No. 2,801,239, relating to the use of zinc chloride as an esterification catalyst, cited as an advantage that the process minimized the rate of breakdown of the cellulose. U.S. Pat. No. 3,518,249 acknowledged that little interest had been shown in cellulose esters of an extremely low degree of polymerization. More recently it was confirmed that the rate of hydrolysis in cellulose esters is controlled by temperature, catalyst concentration, and, to a lesser extent, by the amount of water, and that higher water content slightly increases the rate of hydrolysis and “helps minimize degradation.” Kirk-Othmer, Encyclopedia of Chemical Technology, Fourth Ed., vol. 5, pp. 496-529, 509 (1993), John Wiley & Sons, New York, N.Y.
When used in coating compositions, conventional cellulose esters provide many benefits, including improved hardness, improved aluminum flake orientation, high clarity, high gloss, decreased dry-to-touch time, improved flow and leveling, improved redissolve resistance, reduced cratering, and reduced blocking. However, the performance properties of conventional cellulose esters are accompanied by an increase in viscosity, which must be offset by increasing the level of solvents used. With recent concerns of VOC levels in coating compositions, there remains a need for a cellulose ester product that provides the benefits of conventional cellulose esters, while providing only a moderate increase in viscosity without the addition of organic solvents. It would clearly be an advance in the art to provide cellulose esters that provide the performance properties of conventional cellulose esters, without an undue increase in viscosity when incorporated into coating compositions.
Although maintaining the molecular weight of cellulose esters during esterification and partial hydrolysis has long been deemed important in obtaining a suitable product, there has nonetheless been occasional mention in the literature of lower molecular weight cellulose esters.
For example, U.S. Pat. No. 3,386,932 discloses a method for reducing the molecular weight of cellulose triacetate with a catalyst such as boron trifluoride, the resulting bifunctional, low molecular weight cellulose triacetate then being used to produce linear block copolymers. This disclosure emphasizes the importance of maintaining the ester substitution at the 2-, 3-, and 6-positions of the triacetate, that is, wherein substantially all of the hydroxyl groups of the cellulose have been esterified, so that the hydroxyl functionality necessary for formation of the linear block copolymers preferentially appears only on the ends of the polymer chains.
U.S. Pat. No. 3,391,135 discloses a process in which hydrogen halides are used to reduce the molecular weight of cellulose derivatives. The examples describe methylcellulose powder and methyl-hydroxypropyl cellulose reacted with hydrogen chloride to reduce the molecular weight, as evidenced by a reduction in viscosity.
U.S. Pat. No. 3,518,249 describes oligosaccharide tripropionates, having an average degree of polymerization of from about 4 to about 20 and low levels of hydroxyl, that are useful as plasticizers and as control agents for the manufacture of foamed plastics. The oligosaccharide tripropionates are prepared by degrading a cellulose propionate in the presence of an acid catalyst. The patentees acknowledge that it has been an object in the art to provide methods of preventing the degradation of cellulose esters into low-viscosity oligosaccharide esters.
U.S. Pat. No. 4,532,177 describes base coat compositions that include a film-forming resin component, selected from alkyd, polyester, acrylic and polyurethane resins, from 1.0 to 15.0% by weight pigment, and from 2.0% to 50.0% by weight of a cellulose ester material. The '177 patent suggests a solution viscosity for the cellulose ester material from 0.05-0.005 seconds, an acetyl content from 10.0-15.0% by weight, a propionyl content from 0.1-0.8% by weight, a butyryl content from 36.0-40.0% by weight, and a free-hydroxyl content of from 1.0-2.0% by weight. However, the examples of the patent use a cellulose ester having a solution viscosity of 0.01, which is approximately equivalent to an inherent viscosity (IV) for such an ester of from about 0.25 to about 0.30 dL/g, as measured in a 60/40 (wt./wt.) solution of phenol/tetrachloroethane (PM95) at 25° C.
WO 91/16356 describes a process for the preparation of low molecular weight, high-hydroxyl cellulose esters by treating a cellulose polymer with trifluoroacetic acid, a mineral acid, and an acyl or aryl anhydride in an appropriate carboxylic solvent, followed by optional in situ hydrolysis. The cellulose esters obtained according to the disclosure are said to have a number average molecular weight (Mn) ranging from about 0.01×105 (about 1,000) to about 1.0×105 (about. 100,000), and an IV (inherent viscosity) from about 0.2 to about 0.6, as measured at a temperature of. 25° C. for a 0.25 gram sample in 100 ml of a 60/40 by weight solution of phenol/tetrachloroethane.
Japanese Kokai Patent Publication No. 51-119089 describes a process for the preparation of a low molecular weight cellulose mixed organic acid ester that involves heating cellulose acetate with a saturated or unsaturated organic acid of 3 or more carbon atoms (propionyl or higher), in the presence of an acid catalyst, with removal of the resulting acetic acid from the reaction mixture, to obtain a lower molecular weight cellulose mixed organic acid ester. The starting material for this process is cellulose acetate.
Another patent document naming the same inventors, Japanese Kokai Patent Publication No. 51-119088, discloses a method for the manufacture of a low molecular weight cellulose organic acid ester that includes heating cellulose acetate with a saturated or unsaturated organic acid at a temperature above 30° C. in the presence of a cation exchange resin, the resulting ester having a lower molecular weight than the starting material. The starting material for the disclosed process is cellulose acetate.
Both of these references teach low molecular weight mixed cellulose esters. The process uses cellulose acetate as starting material, and performs a transesterification while hydrolyzing the cellulose backbone, the amount of higher mixed ester introduced being relatively low.
U.S. Pat. No. 6,303,670 discloses an ultraviolet-curable cellulosic coating composition comprising a cellulose acetate, a diepoxy compound, and a photo cationic polymerization catalyst. The cellulose acetate useful in these compositions is a low molecular weight cellulose acetate, having a number-average molecular weight of from 1,500 to 5,000, and is prepared from cellulose triacetate by hydrolysis. According to this disclosure, the degree of substitution of hydroxyl groups must be from 1 to 3, since hydroxyl values of less than 1 are said to result in insufficient crosslinking in the final coating composition.
Although efforts have been made to prepare oligosaccharides via stepwise addition of anhydroglucose units, these methods are not believed to result in cellulose derivatives that are suitable for coating applications. Further, the costs of such processes would be significant. See, for example, Nishimura, T.; Nakatsubo, F. “Chemical Synthesis of Cellulose Derivatives by a Convergent Synthetic Method and Several of Their Properties,” Cellulose, 1997, 4, 109. See also Kawada, T.; Nakatsubo, F.; Umezawa, T.; Murakami, K.; Sakuno, T. “Synthetic Studies of Cellulose XII: First Chemical Synthesis of Cellooctaose Acetate,” Mokuzai Gakkaishi 1994, 40(7), 738.
Cellulose derivatives other than conventional cellulose esters have also been made and used. Maintaining molecular weight has typically been an important aspect in maintaining the desired performance properties in these esters also.
For example, U.S. Pat. No. 4,520,192 describes a process for preparing carboxyalkyl acetyl celluloses with a carboxyalkyl degree of substitution per anhydroglucose unit (DS) of 0.2-2.5 and an acetyl DS of 0.5-2.8 and the metal salts of these materials. The materials are said to be useful as enteric coating agents, in view of their solubility in organic solvents.
U.S. Pat. No. 3,435,027 describes a base catalyzed (sodium hydroxide), process for partially esterifying carboxymethylcellulose (in the sodium salt form) in 70-85% acetone nonsolvent media with acetic, propionic, lactic, or stearyl anhydride. The procedure yields a product with a low degree of ester substitution.
USSR Patent 612933 describes a process for preparation of cellulose acetate ethers wherein an alkali activated cellulose (12% sodium hydroxide) is etherified with monochloroacetic acid followed by esterification with acetic anhydride in the presence of sulfuric acid catalyst. The process is limited to a low DS of the carboxymethyl substituent.
U.S. Pat. No. 3,789,117 discloses a process for preparing an enteric medicament coating from an organic solvent-soluble cellulose derivative. The cellulose derivative's substitution has a carboxymethyl DS range of 0.3 to 1.2, in conjunction with at least one of the remaining hydroxyl groups being etherified or esterified. Ester groups include acetyl, propionyl, butyryl, nitric, or higher fatty acids. Higher fatty acid ester derivatives such as carboxymethylcellulose stearic acid ester derivative are said to be preferred.
RO 96929 describes a carboxymethylcellulose (CMC) acetate with a carboxymethyl degree of substitution of 0.5-3.0, acetyl DS of 0.4-2.9, and viscosity 150-1500 cP. This material was taught to be useful as an alkaline suspension stabilizer, solution thickener, and alkaline media binder.
U.S. Pat. No. 5,008,385 reports cellulose derivatives, that can be synthesized by homogeneous reaction in dimethylacetamide and/or N-methylpyrrolidine containing LiCl, and that have a degree of polymerization of at least 400. Included in their examples was CMC acetate (carboxymethyl DS 0.13-0.23, acetyl DS 2.54-2.15). These materials were tested for use in the production of fibers, filaments, or membranes.
Carbohydrate Research, 13, pp.83-88, (1970) describes the preparation of CMC acetate by sulfuric acid catalyzed acetylation of CMC (carboxymethyl DS of 0.07), hydrolysis (acetylation and hydrolysis procedures taken from Maim, Ind. Eng. Chem., 38 (1946) 77), and evaluation of this material in membranes for reverse osmosis.
Holzforschung, 27(2), pp. 68-70, (1973) describes the rate of carboxymethylation and deacetylation of cellulose acetate in the presence of sodium hydroxide. This work showed that deacetylation and carboxymethylation occur simultaneously with the rate of deacetylation being faster than the rate of carboxymethylation. The highest carboxymethyl DS obtained was less than 0.1.
GB 2,284,421 discloses carboxymethylcellulose alkanoates which are “lightly carboxymethylated”, i.e., having a degree of substitution per anhydroglucose unit of less than 0.2. The resulting products are said to be useful as coating agents for paper and papermaking or as bonding agents for non-woven fabric, or can be extruded to form filaments or film, or can be used to produce shaped articles or a sponge. The products can be dissolved in aqueous sodium hydroxide solutions, but the patentees caution that higher concentrations of sodium hydroxide may tend to degrade the cellulose, leading to a reduction in the desired mechanical properties.
U.S. Pat. Nos. 5,668,273 and 5,792,856 disclose carboxymethylcellulose esters of higher acids, having inherent viscosities from 0.20 to 0.70 dL/g, preferably from 0.35 to 0.60 dL/g, as measured in a 60/40 (wt./wt.) solution of phenol/tetrachloroethane at 25° C., that are useful in coating compositions as binder resins and rheology modifiers.
U.S. Pat. No. 5,994,530 discloses carboxymethylcellulose esters of higher acids, having inherent viscosities from 0.20 to 1.7 dL/g, as measured in a 60/40 (wt./wt.) solution of phenol/tetrachloroethane at 25° C, that are useful in aqueous pigment dispersions.
WO 02/036637 discloses the use of carboxymethylcellulose acetate butyrate for improving holdout on a coated, cellulosic fiberboard substrate.
WO 01/035719 discloses the use of carboxymethylcellulose acetate butyrate in a wood stain formulation to provide good adhesion under an overcoat.
There remains a need in the art for cellulose esters that provide suitable properties for coating compositions, and the like, which do not unduly increase the viscosity of the compositions, and which are furthermore dispersible in water.